For example, in an insulation type DC-DC converter in which an input side and an output side are insulated from each other, a first conversion circuit that converts a DC voltage of a DC power supply into an AC voltage by switching of the DC voltage is provided on the input side, and a second conversion circuit that converts the AC voltage converted with the first conversion circuit into a DC voltage by rectification of the AC voltage is provided on the output side. The first conversion circuit and the second conversion circuit are insulated from each other using a transformer.
There are insulation type DC-DC converters called a boost half bridge system (hereinafter, referred to as a BHB system) in which a boosting chopper (boost converter) and a half bridge type DC-DC converter are combined. U.S. Patent Publication No. 2014/0268908 (Patent Literature 1), Unexamined Japanese Patent Publication No. 2002-315324 (Patent Literature 2), Unexamined Japanese Patent Publication No. 2003-92876 (Patent Literature 3), Unexamined Japanese Patent Publication No. 2003-92877 (Patent Literature 4), Unexamined Japanese Patent Publication No. 2003-92881 (Patent Literature 5), Unexamined Japanese Patent Publication No. 2007-189835 (Patent Literature 6), Unexamined Japanese Patent Publication No. 2007-236155 (Patent Literature 7), Unexamined Japanese Patent Publication No. 2007-236156 (Patent Literature 8), Unexamined Japanese Patent Publication No. 2008-79454 (Patent Literature 9), Unexamined Japanese Patent Publication No. 2010-226931 (Patent Literature 10), and Unexamined Japanese Patent Publication No. 2002-354814 (Patent Literature 11), Shuai Jiang, Dong Cao, Fang Z. Peng and Yuan Li “Grid-Connected Boost-Half-Bridge Photovoltaic Micro Inverter System Using Repetitive Current Control and Maximum Power Point Tracking”, 5-9 Feb. 2012, 2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC), pp. 590-597 (Non Patent Literature 1), Dong Cao, Shuai Jiang, Fang Z. Peng and Yuan Li “Low Cost Transformer Isolated Boost Half-bridge Micro-inverter for Single-phase Grid-connected Photovoltaic System”, 5-9 Feb. 2012, 2012 Twenty-Seventh Annual IEEE Applied Power Electronics Conference and Exposition (APEC), pp. 71-78 (Non Patent Literature 2), Hossein Tahmasebi, “Boost Integrated High Frequency Isolated Half-Bridge DC-DC Converter: Analysis, Design, Simulation and Experimental Results”, 2015 A project Report Submitted in Partial Fulfillment of the Requirements for the Degree of MASTER OF ENGINEERING, University of Victoria (https://dspace.library.uvic.ca/bitstream/handle/1828/6427/Tahmasebi_Hossein_MEng_2015.pdf) (Non Patent Literature 3), and York Jr, John Benson, “An Isolated Micro-Converter for Next-Generation Photovoltaic Infrastructure” 2013-04-19 Dissertation submitted to the Faculty of the Virginia Polytechnic Institute and State University (https://vtechworks.lib.vt.edu/bitstream/handle/10919/19326/York_JB_D_2013.p df) (Non Patent Literature 4) disclose BHB-system insulation type DC-DC converters.
In the BHB-system insulation type DC-DC converter, a main switching element, an auxiliary switching element, an inductor, a primary winding of a transformer, and two capacitors are provided in the input-side first conversion circuit. The inductor and the main switching element are connected in series to the DC power supply, and a series circuit of the primary winding of the transformer and one of the capacitors is connected in parallel to the main switching element. A series circuit of the other capacitor and the auxiliary switching element is connected in parallel to the primary winding of the transformer.
For example, a circuit including two rectifying elements, two capacitors, and a secondary winding of a transformer as illustrated in FIG. 11 of Patent Literature 1 or a circuit including two rectifying elements, one capacitor, one inductor, and a secondary winding of a transformer having a center tap as illustrated in FIG. 1 of Patent Literature 2 is provided in the output-side second conversion circuit.
The main switching element and auxiliary switching element of the first conversion circuit are alternately turned on with a predetermined duty. The auxiliary switching element is turned off in a period in which the main switching element is turned on, and the main switching element is turned off in a period in which the auxiliary switching element is turned on. When the main switching element is turned on, the voltage at one of the capacitors is applied to the primary winding of the transformer, and power is transferred to the secondary winding of the transformer. At this point, the voltage at the primary winding is equal to the input voltage. On the other hand, when the auxiliary switching element is turned on, the voltage at the other capacitor is applied to the primary winding of the transformer, and power is transferred to the secondary winding of the transformer. At this point, the voltage at the primary winding depends on the input voltage and the duty.
In an insulation type DC-DC converter for vehicle, the input voltage is supplied from a battery mounted on a vehicle, and the battery is also used as a power supply of a starter motor that starts an engine. Therefore, for example, when the engine is restarted by release of an idling stop state of the vehicle, a large current is passed through the starter motor, and a large drop in voltage is temporarily generated in the battery. The power for the controller that controls switching operation of the DC-DC converter is also supplied from the battery. Therefore, when the battery voltage is lower than a voltage necessary for operation of the controller, a CPU constituting the controller is reset, and the switching operation becomes uncontrollable.
For example, in Patent Literature 5, a tertiary winding is provided in the transformer of the DC-DC converter, a rectification circuit in which four diodes are bridge-connected to one another is connected to the tertiary winding, and an output voltage at the rectification circuit is smoothed and used as the power for a control circuit. However, in Patent Literature 5, a structure of the transformer becomes complicated because the tertiary winding is provided in the transformer.
There is also a method for obtaining the power for the controller from a boosted secondary-side output voltage. However, in this case, it is necessary that the power be supplied to the primary-side controller while the secondary-side output voltage is insulated. Therefore, it is necessary to provide an insulation circuit between the output terminal and the controller, and the configuration becomes complicated.